Light-emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
Most light sources that are candidates for replacement by LEDs require a plurality of LED dies to provide sufficient light to match the light output of the device being replaced. A replacement light source typically includes a plurality of LED dies, a power supply that converts AC power to DC power and some form of wiring matrix that contacts the plurality of dies in a parallel or serial configuration to the DC power source.
Initial cost and electrical conversion efficiency, and replacement costs are important factors in the design of such a replacement light source. The initial cost depends on the packaging costs inherent in connecting a large number of dies to a substrate and to the power supply. These costs are a significant fraction of the initial cost of an LED replacement for a conventional light source. The initial cost of the light source also depends on the degree to which the manufacturer of the light source must build each configuration from scratch by connecting individual LEDs to a substrate and controller that are particular to that configuration. There are a large number of light source configurations that must be produced to compete with conventional lighting technology. Each configuration is characterized by a total light output and a generated light spectrum. Even for “white” light sources, there is a range of “color temperatures” that typically vary from cool white to warm white. Other useful configurations provide the ability to dim the light source or change its color temperature after installation to vary the “mood” of the space being illuminated.
The long-term costs associated with the light source depend on the electrical conversion efficiency, the lifetime of the light source, and the cost of the replacement of the light source. LEDs have lifetimes that are significantly greater than those of conventional light sources. Hence, a light source based on LEDs has the potential of outlasting conventional light sources, and hence, reducing the cost of replacement. In many applications, the cost of replacement is many times the cost of the light source. While individual LEDs have very long lifetimes, a light source having tens of LEDs that are connected to a substrate and other components, has a significantly shorter time to failure. Hence, a high reliability light source must provide some mechanism for continued operation even when one or more of the LEDs or the connections thereto fail.
The electrical conversion efficiency depends on both the temperature and the amount of current that is driven through the LEDs. An LED can be modeled as a resistor in series with an ideal diode. The light output from the diode increases with increasing current; however, the power dissipated in the resistor increases as the square of the current. Hence, as the current increases, a greater fraction of the energy is dissipated as heat. As the temperature of the LED increases, the efficiency and lifetime of the LED decreases. As a result, a light source having a large number of smaller LEDs provides better efficiency than a light source having a fewer number of LEDs that are driven at higher currents. However, the increased number of LEDs also increases the packaging costs and the probability of failure due to one of the LEDs or its connections failing.